Here. I searched up "goth necklace" on Etsy, and I didn't know your price point, but I set it at $50 max, and here are the results:
https://www.etsy.com/search?q=goth necklace&order=most_relevant&min=&max=50
For specific shops, look into The Rogue and the Wolf:
https://www.etsy.com/shop/TheRogueAndTheWolf?ref=shop_sugg
and The Crypt of Curiosities:
https://www.etsy.com/shop/TheCryptOfCuriositie?ref=related-shop
For steampunk, I searched "steampunk accessories" and got this, again with the $50 max price limit:
https://www.etsy.com/search?q=steampunkaccessories&order=most_relevant&min=&max=50
I did the same with "rockabilly":
https://www.etsy.com/search?q=rockabillyaccessories&order=most_relevant&min=&max=50
Clothing is probably going to be the most expensive thing to buy, and not knowing your price point, I didn't really search it up. But you can just add in search terms like "shirt" or "skirt" if you wanted to look up clothing options. After browsing a bit and finding an idea or things you like, you can further narrow down your search by adding those terms and finding more specific things. If she likes certain goth themes, try searching "bat necklace" or "skull ring" or something like that. Etsy works best when you have a fairly defined idea of what you're looking for, as it searches what the seller has tagged the item. Searching up just "goth" will just overwhelm you with options and won't get you the best stuff.